Recently, active research has been directed to lithium ion batteries and fuel cells used as power sources for mobile appliances, and to solid electrolyte membranes constituting the above batteries or cells. The solid electrolyte membranes are, for instance, lithium ion conductive materials and proton conductive materials.
Generally, the proton conductive material is formed in a membrane form. The solid electrolyte in the membrane form used as a solid electrolyte layer of the battery or the cell such as the fuel cell, and a producing method thereof are suggested in the following. For instance, Japanese Patent Laid-Open Publication No. 9-320617 suggests a method in which polyvinylidene fluoride resin is immersed into a liquid mixture of an electrolyte and a plasticizer. Japanese Patent Laid-Open Publication No. 2001-307752 suggests a producing method of a proton conductive membrane by synthesizing an inorganic compound in a solution containing aromatic polymer having sulfonic acid group, and then removing the solvent. In this method, oxides of silicon and phosphoric acid derivative are added to improve shapes and conditions of micropores. Japanese Patent Laid-Open Publication No. 2002-231270 suggests a method for producing an ion exchange membrane by adding a metal oxide precursor to a solution containing ion exchange resin, and then casting a liquid obtained by hydrolysis and polycondensation of the precursor. Japanese Patent Laid-Open Publication No. 2004-79378 suggests a producing method of the proton conductive membrane. First, a polymer membrane having proton conductivity is produced by a solution casting method. To produce the proton conductive membrane, the above polymer membrane is immersed in a water-soluble organic compound solution whose boiling point is not less than 100° C. to reach equilibrium swelling, and then the water is evaporated by heating. Japanese Patent Laid-Open Publication No. 2004-131530 suggests a producing method of a solid electrolyte membrane by dissolving a compound whose main component is polybenzimidazole having negative ionic group in an alcoholic solvent containing tetraalkylammonium hydroxide and whose boiling point is not less than 90° C.
As the membrane forming method, there are a melt extrusion method and a membrane casting method as well known. In the former method, the membrane is produced without using the solvent. However, the polymer is denatured due to heating, and impurities in the polymer material remains in the membrane. On the other hand, the latter method requires a large sized facility including a producing apparatus of the solution which is called a dope, a solvent recovery device and the like. However, the latter method only requires low heating temperature, and enables to remove the impurities in the polymer material. Furthermore, in the latter method, a membrane with superior flatness and smoothness is produced compared to the membrane produced by the former method.
In Japanese Patent Laid-Open Publication No. 9-320617, the solution casting method is rejected, but the problem of impurities contained in the raw material remaining in the membrane is not solved. The producing methods disclosed in Japanese Patent Laid-Open Publications No. 2001-307752, 2002-231270, 2004-79378, and 2004-131530 are for small-scale productions and not for large scale manufactures. The method disclosed in Japanese Patent Laid-Open Publication No. 2001-307752 has a problem in that dispersion of complex made of a polymer and an inorganic solvent is difficult. The method disclosed in Japanese Patent Laid-Open Publication No. 2002-231270 has a problem in that the membrane production process is complicated. In a method disclosed in Japanese Patent Laid-Open Publication No. 2004-79378 has a problem in that micropores are formed on the membrane by immersing the membrane in the water. As a result, the uniform membrane is not obtained. A method for solving the above problem is not disclosed. Further, the above reference cites that the method enables to produce various kinds of solid electrolyte membranes. However, concrete disclosure is not given. In Japanese Patent Laid-Open Publication No. 2005-146018 limits the materials to be used in the method so that other superior materials cannot be used.
None of the above references disclose a method for forming a solid electrolyte multilayer membrane which imparts desired functions to the solid electrolyte membrane.
An object of the present invention is to provide a solid electrolyte membrane with excellent proton conductivity in a continuous membrane form with the constant quality, a method and apparatus for producing the same, and the membrane electrode assembly and the fuel cell using the solid electrolyte membrane.